Marriage has long been, and continues to be to this day, the only form of gender relations recognized as socially legitimate. This monopoly position has only been shaken and partly broken in the more developed countries in our days. It is therefore particularly interesting today to consider why it was considered essential in earlier times that the relationship between men and women should be publicly legitimized, and above all which authorities claimed to legitimize a gender relationship as a marriage and had the power to enforce recognition of this claim. These are the questions at the heart of this book. The perspective of Schröter's investigation explicitly differs from legal and especially canon law traditions, to whose domain the topic belongs, at least in German.